Rational design of supramolecular systems for application in photonic devices requires a clear understanding of both the mechanism of energy and electron transfer processes and how these processes can be manipulated. Central to achieving these goals is a detailed picture of their electronic structure and of the interaction between the constituent components. We review several approaches that have been taken towards gaining such understanding, with particular focus on the physical techniques employed. In the discussion, case studies are introduced to illustrate the key issues under consideration.

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Elucidating excited state electronic structure and intercomponent interactions in multicomponent and supramolecular systems

Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi -systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a Forster mechanism. In contrast, systems with a rr-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi -conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.

Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems.

Binuclear zinc(II) and copper(II) complexes based on a new Schiff base ligand N,N′-bis(2-hydroxybenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H2L) have been synthesized. The ligand H2L and complexes under investigation have been characterized by elemental analyses, spectral (FT-IR, 1H, 13C NMR, ESI-MS, electronic absorption, emission), and electrochemical studies. The structures of H2L and complexes [{Zn(C23H18N2O2)}2] (1) and [{Cu(C23H18N2O2)}2]·H2O (2) have been determined crystallographically. Selective “On-Off-On” switching behavior of the fluorescent complex 1 has been studied. The fluorescence intensity of 1 quenches (turns-off) upon addition of Cu2+, while enhances (turns-on) in the presence of Ag+ ions. The mechanisms of “On-Off-On” signaling have been supported by 1H NMR, ESI-MS, electronic absorption, and emission spectral studies. Job’s plot analysis supported 1:1 and 1:2 stoichiometries for Cu2+ and Ag+ ions, respectively. Association and quenching constants have been estimated by the Benes...

Fluorescent zinc(II) complex exhibiting "on-off-on" switching toward Cu2+ and Ag+ ions.

Delocalisation of photoexcited triplet states probed by transient EPR and hyperfine spectroscopy.

113 – Recent Advances in Free and Metalated Multiporphyrin Assemblies and Arrays; A Photophysical Behavior and Energy Transfer Perspective

Spacer dependence of intersystem crossing (ISC) affected by a remote metal unpaired electron was studied in a series of copper(II) porphyrin−free base porphyrin dimers. In these dimers, fluorescence of the free base moiety is remarkably quenched by the copper counterpart which is linked via a phenanthrene, a naphthalene, or a benzene. Fluorescence lifetimes and quantum yields of the free base moiety in the heterodimers are 1/10−1/35 of the free base monomer depending upon the spacer, whereas no change was observed in the free base homodimers. The changes in the copper(II)−free base dimers are attributed to an enhancement of ISC in the free base half, due to interaction with an unpaired electron in Cu(II) ion of the other half. Estimated ISC rates from the experimental results exhibit a strong correlation with the number of bonds of the linkage but not with the center-to-center distance of the two halves. An expression for the ISC rate in the presence of an unpaired electron is derived. Exchange interactio...

Enhanced Intersystem Crossing in Gable-Type Copper(II) Porphyrin−Free Base Porphyrin Dimers: Evidence of Through-Bond Exchange Interaction

Triplet−triplet intramolecular energy transfer in a covalently linked copper(II) porphyrin−free base porphyrin hybrid dimer was examined by time-resolved (TR) ESR measurements of the lowest excited triplet (T1) state of the free base porphyrin component after laser pulse excitation at 532 nm. The TRESR spectra of the free base moiety in the hybrid dimer in toluene glass at 77 K exhibit a spin polarization pattern different from that for the free base porphyrin monomer. The observed pattern for the dimer cannot be explained by any intersystem crossing (ISC) process, and it is ascribed to intramolecular energy transfer that takes place from the copper porphyrin part to the free base counterpart between the triplet manifolds. On the other hand, the TRESR spectrum in 2-methyltetrahydrofuran (2-MTHF) glass, where the energy transfer is prohibited by the fast deactivation of the excited copper porphyrin, shows the same spin polarization pattern as that of the free base porphyrin monomer. Therefore, the copper p...

Takatoshi Ichino

Papers

Enhanced Intersystem Crossing in Gable-Type Copper(II) Porphyrin−Free Base Porphyrin Dimers: Evidence of Through-Bond Exchange Interaction

Triplet−Triplet Intramolecular Energy Transfer in a Covalently Linked Copper(II) Porphyrin−Free Base Porphyrin Hybrid Dimer: A Time-Resolved ESR Study

A time-resolved ESR study on triplet-triplet energy transfer processes in a copper(II)-free-base porphyrin dimer